This invention relates to printing machines, and more particularly to a method and apparatus for adjusting for toner concentration drifts in such a printing machine.
In electrostatographic copiers and printers, toner reproductions are made using toner particles, contained in developer material at a desired concentration level. As toner particles are depleted from the developer material, additional toner particles must be added thereto in order to maintain the toner concentration at the desired level. Typically, the toner concentration of a machine is monitored by suitable means, and is maintained by adding fresh toner particles to the development housing of the machine.
For monitoring and maintaining the toner concentration of such a machine, many types of systems including high cost toner concentration sensors, have been proposed. For example, U.S. Pat. No. 4,619,522 to Imai teaches the use of a reference pattern, with a predetermined reflectance, that is developed. Subsequently, the density of the developed pattern is detected by a sensor, and used to regulate the replenishment of toner to the developer housing.
Furthermore, U.S. Pat. No. 4,434,221 to Oka discloses a method of utilizing a reference latent image to measure the current flow between the developing sleeve and the photoreceptor drum during development of the reference image. Subsequently, the amount of toner needed for replenishment is controlled, based on the current value measured. Oka further characterizes this method as inferior, because, the variation in current value due to toner concentration is exceeded by the variation due to the amount of toner adhering to the reference image.
In addition, U.S. Pat. No. 4,492,179 to Folkins et al., teaches the sensing of the charge of the toner particles being transferred to the latent image, and means for controlling the addition of toner to the developer housing as a function of that measurement. Folkins et al. also discloses the limitations of the marking particle dispense control system, relating to toner dispensing assumptions, in which the rate of dispense must remain constant over the life of the system. More specifically, any variation in the toner mass dispensed for a given electrical input will manifest itself proportionally as a shift in the relationship between the toner dispense rate and the bias current required for the developed toner charge.
Unfortunately however, these proposed systems can each be prohibitively expensive particularly with respect to low volume printing machines. Ordinarily, each such low volume, low cost machine will tend to use a low cost toner concentration maintenance and control system. Such a low cost system, for example, can involve the estimation of an amount of toner to be depleted by each toner reproduction to be made, as well as, the automatic addition or replenishment, after reproduction, of an amount of fresh toner equal to the estimated depletion amount. However, because of the low cost nature of such a control system, and because of the potential for, and actual, errors both in estimation and replenishment of toner amounts, the actual toner concentration achieved may tend to drift one way or the other about a desired nominal level. This is because such errors or, variations tend to result in a higher or lower than expected average toner feed rate. The variations or errors in image area coverage sensing accuracy are due, for example, to effects of dirt accumulation on the sensor 31. All in all, it has been found that up to a 10% variation can occur in toner amounts fed. Such variations in toner feeding accuracy, as expected, tend to cause the machine to drift out of an acceptable toner concentration operating range, and thus result in copy quality deficiencies.
it is therefore common in such machines to enable an operator based on inspection of toner reproductions, to make manual adjustments to the amount of fresh toner actually added by the system. In such machines, errors in over feeding of toner are more damaging and harder for an operator to control than errors of under feeding toner. The control system may therefore be preferably and intentionally set to feed toner slightly on the low side, thereby causing the toner concentration of the machine to slowly drift downwards. When it eventually drifts too low, as judged by the operator from inspecting toner reproductions, the operator can press a "toner-add button" so as to add additional toner to boost the toner concentration back up. Equally however, a "toner-reduce button" can also be provided for manually reducing amounts of fresh toner that would otherwise be added automatically.
Such operator adjustments, however, can unfortunately make the problem worse by resulting in wider and varying drifts about the desired or nominal concentration level. In addition, frequent and too many, such adjustments by an operator can detrimentally affect perceived customer satisfaction with the performance of the machine, even if toner reproductions made following such adjustments are actually of acceptable quality.
There is therefore a need to provide a method and apparatus in such low volume, low cost machines for effectively controlling the toner concentration thereof while minimizing the number and frequency of such operator adjustments.